The present invention relates to a power transmission shaft used in apparatuses such as automobiles and industrial machines to transmit torque via a constant velocity joint. The present invention also relates to a constant velocity joint used in apparatuses such as automobiles and industrial machines to transmit driving power.
A power transmission shaft, for example the drive shaft of an automobile, is usually made of carbon steel or carburized steel, and is ensured to have a specified strength by setting proper surface hardening and effective case depth achieved by a heat treatment.
Recently, as the automobiles tend to have increasing output power and the vehicle weight increases for higher safety requirements, the drive shaft is required to have higher strength. On the other hand, the drive shaft is required to be lighter in weight in order to improve the fuel efficiency, that also imposes a pressing need to increase the strength of the drive shaft.
In order to increase the load capacity of the shaft, it is common to increase the carbon content of the material thereby to achieve a higher material strength or increase the effective depth of hardened layer (case depth). However, the former approach leads to decreased strength in notched parts, and lower workability, such as the ease of forging and cutting, due to the increased hardness of the material. The latter approach, on the other hand, leads tot very narrow range of case depths that can be obtained in the case of carburized steel. Also in the case of a shaft made of carbon steel, it becomes more difficult to apply deep hardening as the shaft diameter increases, and it is very difficult to carry out deep hardening with the ratio of effective case depth to shaft radius (hereinafter denoted xcex3) higher than 0.4 since it may lead to defects such as quenching crack. Recently carbon steel that contains boron B added has been used to enable deep hardening. However, even though the effective case depth is increased with the use of this material, only an increase in the strength up to about 15% is possible since the static strength and the torsional fatigue strength reach the plateau at xcex3 greater than 0.65 and xcex3 greater than 0.5, respectively (Japanese Patent Application Laid-open No. Hei 5-320825). Also in the case of a material with B added, hard nitrogen compounds such as TiN are formed that may lead to lower cutting workability.
The constant velocity joint used in the power transmission shaft falls roughly into two classes of fixed type that allows displacement only in the angle between the two shafts, and sliding type that allows both angular displacement and axial displacement, which are selected according to the operating conditions, purpose and other factors. The fixed type includes the Rzeppa type constant velocity joint and the sliding type includes the double offset type onstant velocity joint and tripod type constant velocity joint as the representative ones.
Applications of the constant velocity joint include the power transmission system of the automobile. Recently, as the automobiles tend to have increasing output power and the vehicle weight increases for higher safety requirements, constant velocity joints of the drive shaft are required to have higher strength. On the other hand, the drive shaft is required to be lighter in weight in order to improve the fuel efficiency, that also imposes a pressing need to increase the strength of the constant velocity joint.
An outer member (outer race) that is a constituent element of the constant velocity joint is made of carbon steel or the like, that is forged into a predetermined form and subjected to heat treatment such as induction hardening in order to ensure the required levels of strength, durability and wear resistance, followed by grinding of portions that require high precision thereby finishing the part to the predetermined dimensions and completing the product. High strength requirement in this case may be satisfied by either increasing the carbon content thereby to increase the material strength or increasing the effective case depth. The former method, however, lowers the machinability for such processes as forging and cutting and leads to increased manufacturing cost. The latter method, on the other hand, is limited in the effect of increasing the strength since the anticipation of defects such as quenching crack makes it difficult to apply further deep hardening.
Constituent elements (inner member, cage, tripod member, etc.) of the constant velocity joint are made of carbon steel or the like, that is machined to a predetermined form and subjected to carburizing treatment in order to ensure the required levels of strength, durability and wear resistance, followed by grinding of portions that require high precision thereby finishing the part to the predetermined dimensions and completing the product.
When a part is carburized by heat treatment, however, the part undergoes a significant deformation caused by the heat treatment with variations in the dimensions. Thus it has been necessary to finish the parts by grinding after the heat treatment. Also pocket surfaces on both sides of the axis among pockets of the cage, for example, must have a certain surface accuracy in order to regulate the positions of torque transmitting balls, but the grinding process after the heat treatment is sometimes omitted in order to simplify the machining process. When the grinding process is omitted, parts that have large deformations caused by the heat treatment are rejected, resulting in higher reject ratio.
Accordingly, an object of the present invention is to provide a power transmission shaft that has high workability for such processes as forging and cutting, and high strength.
Another object of the present invention is to increase the strength of an outer race of a constant velocity joint while simplifying the machining processes for lower manufacturing cost and increase the accuracy.
Further another object of the present invention is to simplify the machining processes for the components of the joint such as the inner member, the cage and the tripod member and cut down on the manufacturing cost of the constant velocity joint.
The power transmission shaft will first be described below.
According to the present invention, in order to achieve the object described above, in the power transmission shaft using the constant velocity joint, graphite steel is subjected to induction hardening thereby to increase the surface hardness, and a 2-phase structure of ferrite and martensite is formed in the core. Graphite steel is made by graphitization annealing to turn the cementite included in the carbon steel into graphite, and such properties as high cutting machinability due to the inclusion of graphite that is a free cutting element and favorable property for cold forging and warm forging due to softness. Consequently, graphite steel maintains high machinability even when treated to include a high concentration of carbon for the purpose of increasing the strength.
While majority of the conventional power transmission shafts have been manufactured by applying induction hardening treatment to carbon steel, the core is not subjected to the influence of heat in many cases in order to avoid defects such as quenching crack. Even in such cases as the core is subjected to the influence of heat, most of the core has turned into martensite and therefore the residual compressive stress on the surface has diminished. According to the present invention, on the contrary, effect of the heat by induction hardening not only hardens the surface layer but also reaches the core to form solid solution of graphitewith ferrite, thereby turning the core into 2-phase structure of ferrite and martensite. As a consequence, residual compressive stress remains on the surface thus making it possible to achieve higher strength and high resistance against fatigue. In order to give the effect of heat treatment to the core, it is preferable to carry out induction hardening a plurality of times (for example, twice).
As the graphite steel described above, such a material is used that contains 0.35 to 0.70% of C, 0.4 to 2.0% of Si, 0.3 to 1.5% of Mn, 0.025% or less S, 0.02% or less P, 0.01 to 0.1% of Al, 0.001 to 0.004% of B and 0.002 to 0.008% of N by weight as the basic components, with the rest comprising Fe and inevitable impurities.
Among the elements described above, C is an indispensable element for forming graphite. When the concentration of C is lower than 0.35%, surface hardness after induction hardening becomes too low resulting in insufficient strength. When the concentration of C is higher than 0.70%, cementite precipitates in the core thus making the core harder (brittle) and resulting in lower strength.
Si is added as a deoxidizing agent and graphitization accelerating agent during the steel making process and, in addition, for the purpose of enhancement of grain boundary. When the concentration of Si is lower than 0.4%, it becomes difficult to graphitize the carbide and the effect of grain boundary enhancement decreases. When the concentration of Si is higher than 2.0%, cold workability (ease of forging and cutting by turning) lowers significantly.
Mn content is required to fix sulphur included in the steel in the form of MnS and diffuse it. When the concentration of Mn is lower than 0.3%, hardenability becomes lower (sufficient depth of hardening cannot be obtained). When the concentration of Mn is higher than 1.5%, graphitization is significantly impeded and cold workability lowers.
S, existing in the form of MnS inclusion by bonding with Mn, may be the start point of cracking during cold working, and the concentration thereof is kept 0.025% or less. Concentration of P that precipitates in the grain boundaries of steel thereby to make the grain boundaries brittle, decrease the strength and increase sensitivity to quenching crack, is kept 0.02% or less.
Al, used as a deoxidization agent to remove oxygen included in the steel by being oxidized during steel making process, is contained with a concentration not less than 0.01%. Since a high concentration of oxide lowers the toughness and theoxide may become the start point of crack during cold working, the concentration of Al is kept 0.10% or less.
B and N are added in order to reduce the graphitization annealing time through the generation of BN. While addition of B with a concentration not less than 0.001% is required to achieve sufficient effect of time reduction, the effect of reducing the graphitization annealing time reaches a plateau at a concentration higher than 0.004%. N is added in a concentration in a range from 0.002% to 0.008% inclusive, in order to turn from 0.001% to 0.004% of the B content into BN.
The graphite steel described above includes 0.3 to 1.0 weight percent inclusive of Ni and/or 0.2 weight percent or less Mo added thereto. Addition of Ni increases the ductility of ferrite thereby improving the cold workability and strength. Ni content below 0.3% is insufficient for improving the cold workability and strength, and that higher than 1.0% lowers the turning machinability significantly. Addition of Mo improves the toughness, but content thereof higher than 0.2% impedes graphitization.
Strength can be balanced and prevented from decreasing, when the difference between maximum and minimum values of the surface hardness (Vickers hardness) is 200 Hv or less. Variations in strength of this range can be achieved by using graphite steel wherein the graphite grains are not greater than 15 xcexcm in diameter. When the graphite grain size is greater than 15 xcexcm, voids (pores) generated by the solution of graphite after hardening become larger, leading to soft spots and significant variations in the surface hardness, thus resulting in decreased strength.
The power transmission shaft described above is made to have a core portion that has hardness (Rockwell hardness) in a range from 25 to 45 HRC inclusive. When the hardness is lower than 25, sufficient strength cannot be obtained due to low proportion of martensite. When the hardness is higher than 45, proportion of full martensite increases thus making quenching crack more likely to occur in notches of the shaft such as serrated portions.
Fatigue strength can be improved by maintaining the residual compressive stress in the surface at 60 kgf/mm2 or higher. When induction hardening is applied to graphite steel, hardenability is poor because solid solubility of the graphite portions is low during xcex3 transformation. Consequently, quenching crack is less likely to occur even when subjected to water quenching similarly to high-carbon steel. With water quenching, high surface residual compressive stress of about 60 kgf/mm2 can be achieved.
Fatigue strength can be improved further by applying shot peening after applying induction hardening, thereby to increase the residual compressive stress in the surface to 90 kgf/mm2 or higher. In order to achieve this, it is preferable to apply shot peening twice.
The present invention described above makes it possible to provide a high-strength power transmission shaft that is excellent in machinability for such processes as cutting, cold forging and warm forging, and has high static strength and high fatigue strength.
The present invention also provides a constant velocity joint comprising an outer member that has a plurality of guide grooves formed on the inner circumference thereof, an inner member that has a plurality of guide grooves formed on the outer circumference thereof, torque transmitting balls arranged in a plurality of balls tracks formed from the guide grooves of the outer member and the guide grooves of the inner member, and a cage that holds the torque transmitting balls; or
a constant velocity joint comprising an outer member that has three track grooves formed on the inner circumference thereof and roller guide surfaces disposed in the axial direction on either side of each track groove, a tripod member that has three arms extending radially and rollers rotatably mounted via a plurality of rolling elements on the three arms of the tripod member, the rollers being guided in the axial direction of the outer member by means of the roller guide surfaces on both sides of the track groove, wherein
the outer member is subjected to such treatment as the surface layer is hardened by induction hardening of graphite steel and 2-phase structure of ferrite and martensite is formed in the core.
xe2x80x9cGraphite steelxe2x80x9d is a carbon steel of which cementite contents are turned into graphite by graphitization annealing, and has 2-phase structure of ferrite and graphite. The graphite steel has favorable properties such as high cutting machinability due to the inclusion of graphite that is a free cutting element and advantageous properties for cold forging and warm forging due to softness. Consequently, graphite steel maintains high machinability for such processes as cutting and forging even when treated to include a high concentration of carbon to increase the strength.
While many of the outer members of the prior art have been manufactured by applying induction hardening to carbon steel, the core is not subjected to the influence of heat in many cases in order to avoid defects such as quenching crack. Even in such cases as the core is subjected to the influence of heat, the core has mostly turned into martensite and therefore the residual compressive stress on the surface has diminished. According to the present invention, on the contrary, the effect of heat by the induction hardening not only hardens the surface layer but also reaches the core thereby to form a 2-phase structure of ferrite and martensite in the core. As a consequence, residual compressive stress remains on the surface thus making it possible to achieve higher strength and high resistance against fatigue. In order to give the effect of heat treatment to the core, it is preferable to carry out induction hardening a plurality of times, for example, twice.
As the graphite steel described above, such a material is used that contains 0.5 to 0.70% of C, 0.4 to 2.0% of Si, 0.4 to 1.5% of Mn, 0.025% or less S, 0.02% or less P, 0.01 to 0.1% of Al, 0.001 to 0.004% of B and 0.002 to 0.008% of N by weight as the basic components, with the rest comprising Fe and inevitable impurities.
Among the elements described above, C is an indispensable element for forming graphite. When the concentration of C is lower than 0.50%, surface hardness of the rolling surface after heat treatment becomes too low, and therefore sufficient strength and wear resistance cannot be achieved. When the concentration of C is higher than 0.70%, excessive hardness and precipitation of cementite in the core after the heat treatment result in lower strength.
Si is added as a deoxidizing agent and graphitization accelerating agent during steel making process and, in addition, for the purpose of enhancement of grain boundary. When the concentration of Si is lower than 0.4%, it becomes difficult to graphitize the carbide and the effect of grain boundary enhancement decreases. When the concentration of Si is higher than 2.0%, cold workability (ease of forging and cutting by turning) lowers significantly.
Mn content is required to fix sulphur included in the steel in the form of MnS and diffuse it. When the concentration of Mn is lower than 0.4%, hardenability becomes lower (sufficient depth of hardening cannot be obtained). When the concentration of Mn is higher than 1.5%, graphitization is significantly impeded and cold workability lowers.
S, existing in the form of MnS inclusion by bonding with Mn, may become the start point of cracking during cold working, and therefore the concentration thereof is kept 0.025% or less. Concentration of P, that precipitates in the grain boundaries of steel thereby to significantly lower the hot workability and significantly decrease the material strength, is kept 0.02% or less.
Al, used as a deoxidization agent to remove oxygen included in the steel by being oxidized during steel making process and reduce the particle size, is contained with a concentration not less than 0.01%. Since a high concentration of oxide lowers the toughness and the oxide may become the start point of crack during cold working, the concentration of Al is kept 0.10% or less.
B and N are added to reduce the graphitization annealing time through the generation of BN. While addition of B with a concentration of 0.001% or higher is required to achieve sufficient effect of time reduction, the effect of reducing the graphitization annealing time reaches a plateau at a concentration higher than 0.004%. N is added in a concentration in a range from 0.002% to 0.008% inclusive, in order to turn from 0.001% to 0.004% of B content into BN.
The graphite steel described above includes 0.3 to 1.0 weight percent inclusive of Ni and/or 0.2 weight percent of Mo added thereto. Addition of Ni increases the ductility of ferrite thereby improving the cold workability and strength. Ni content below 0.3% is insufficient for improving the cold workability and strength, while Ni content higher than 1.0% lowers the turning machinability significantly. Addition of Mo improves the toughness, but content thereof higher than 0.2% impedes graphitization.
For the graphite steel, that of graphite grain size within 15 xcexcm is used. When the graphite grain size is greater than 15 xcexcm, voids generated by the solution of graphite after quenching become larger, and soft spots cause the surface hardness to vary significantly, thus lowering the strength, wear property and life related to flaking.
The outer member is formed in a predetermined shape by forging. Forging temperature is set to not higher than the Al transformation temperature (approximately 730xc2x0 C.), in order to prevent carbide represented by cementite from precipitating in the graphite steel structure. This is because, when the temperature is higher than the A1 transformation temperature, precipitation of cementite increases significantly thereby to impede the effect of forging and significantly lowers the machinability (cutting performance) in the subsequent processes. When this temperature condition is satisfied, 2-phase state of ferrite and graphite is maintained even after completing the product, as the precipitation of carbide is regulated at least in the forged skin that remains in the outer member. xe2x80x9cForged skinxe2x80x9d used herein refers to a portion of the product where surface of the structure caused by forging remains, namely product surface left as induction-hardened without being ground, such as the bottom of the mouth of the outer member.
Balanced strength can be maintained and the strength can be prevented from decreasing when the difference between maximum and minimum values of the surface hardness (Vickers hardness) is 200 Hv or less. Variations in strength of this range can be achieved by using graphite steel wherein the graphite grains are not greater than 15 xcexcm in diameter.
The outer member is made to have a core (core of serrated portion) of Rockwell hardness in a range from 25 to 45 HRC inclusive. When the core has hardness lower than 25 HRC, insufficient effect of strength improvement is obtained due to low content of martens ite. When the hardness is higher than 45 HRC, full martensite content increases that makes quenching crack likely to occur in notched portion (serrated portion, for instance) of the shaft. Hardness of the core can be controlled by regulating the processing temperature and duration of induction hardening and carbon content in the graphite steel. It suffices to achieve the required level of hardness described above at least in the core of the serrated portion. Other portions such as the core of mouth are normally made to have higher hardness than the core of the serrated portion.
Improvement in fatigue strength can be achieved when the residual compressive stress in the surface is 50 kgf/mm2 or higher. When induction hardening is applied to graphite steel in general, hardenability is poor because solid solubility of the graphite portions is low during xcex3 transformation. Consequently, quenching crack is less likely to occur even when subjected to water quenching, similarly to high-carbon steel. With water quenching, high residual compressive stress in the surface of about 50 kgf/mm2 can be achieved. It suffices to achieve the required level of residual compressive stress described above at least in the surface of the serrated portion. Surfaces of other portions, for example surface of the mouth, shows higher value of residual compressive stress than the serrated portion.
Fatigue strength can be improved further by applying shot peening after induction hardening thereby to increase the residual compressive stress in the surface to 80 kgf/mm2 or higher. In order to achieve this, it is preferable to carry out shot peening twice. Shot peening is applied at least to the serrated portion and the outer circumference of the mouth.
Wear resistance can be improved by using a low friction grease, specifically a grease having friction coefficient xcexc of 0.07 or lower, for filling the inside of the constant velocity joint. The friction coefficient xcexc can be measured with SAVIN friction wear tester.
The present invention described above makes it possible to provide the outer member that is excellent in machinability for such processes as cutting, cold forging and warm forging, and has high strength such as static strength and fatigue strength. As a consequence, cost reduction and improvement in strength can be achieved for the constant velocity joint.
Also in the present invention, a constant velocity joint is fabricated comprising an outer member that has a plurality of guide grooves formed on the inner circumference thereof, an inner member that has a plurality of guide grooves formed on the outer circumference thereof, torque transmitting balls arranged in a plurality of ball tracks formed from the guide grooves of the outer member and the guide grooves of the inner member, and a cage that holds the torque transmitting balls, wherein either one or both of the cage and the inner member is formed from graphite steel subjected to austempering treatment.
Also in the present invention, a constant velocity joint (tripod type constant velocity joint) is fabricated comprising an outer member that has three track grooves formed on the inner circumference thereof and roller guide surfaces disposed in the axial direction on either side of each track groove, a tripod member that has three arms extending and protruding radially and rollers rotatably mounted via a plurality of rolling elements on the three arms of the tripod member, wherein the rollers are guided in the axial direction of the outer member by means of the roller guide surfaces on both side of the track groove, wherein the tripod member is formed from graphite steel subjected to austempering treatment.
Further in the present invention, a constant velocity joint is fabricated comprising an outer member that has guide grooves of curved shape formed on spherical inner circumference thereof, an inner member that has guide grooves of curved shape formed on spherical outer circumference thereof, torque transmitting balls arranged in a ball track formed from the guide grooves of the outer member and the guide grooves of the inner member, and a cage that holds the torque transmitting balls, wherein center of the guide grooves of the outer member and center of the guide grooves of the inner member are offset to the opposite sides in the axial direction by the same distance with regards to the center plane of the joint that includes the centers of the torque transmission balls, the ball track being gradually reduced toward the opening or inner end of the joint, and the torque transmitting balls being elastically pressed toward the reduced side of the ball track, wherein the outer member being formed from graphite steel subjected to austempering treatment.
xe2x80x9cGraphite steelxe2x80x9d is a carbon steel of which cementite contents are turned into graphite by graphitization annealing, to form 2-phase structure of ferrite and graphite, and has favorable properties such as high cutting machinability due to the inclusion of graphite that is a free cutting element and advantageous property for cold forging and warm forging due to softness consequently, graphite steel maintains high machinability even when treated to include a high concentration of carbon in order to increase the strength.
Austempering is one type of hardening process that utilizes the S curve in the phase diagram of steel. It is a heat treatment process wherein steel heated into the austenite region is immersed in a hot bath (a bath of salt or lead-bismuth) that is maintained at the banite forming temperature, namely in a range of temperatures between Arxe2x80x2 and Arxe2x80x3 transformation points below the knee of the S curve (the lowest temperature at which the transformation takes place), and held therein until the steel structure turns completely to banite, before being taken out therefrom and cooled down to the room temperature. When the steel is held at a high bath temperature, upper bainite having feather-like structure is formed and, at temperatures near Ms point, lower bainite having rod-like structure is formed. Bainite structure is basically a mixture of ferrite and iron carbide, which has a mechanical property that is said to be tougher than a structure of the same hardness achieved by hardening and annealing.
When austempering treatment is applied to carbon steel in order to increase the hardness to 50 HRC (Rockwell hardness) or higher, it requires high carbon content that significantly lowers the workability of the material for forging and other processes.When austempering treatment is applied to graphite steel as in the case of the present invention, workability for forging can be improved due to the ductility (lower resistance against deformation) of the graphite steel. Also because austempering treatment causes far less thermal deformation than other hardening processes, grinding process after the heat treatment can be omitted. Furthermore, since annealing is not required, the cost of heat treatment can be made lower than the conventional heat treatment (hardening plus annealing). Consequently, processes for manufacturing the components of the constant velocity joint, namely the cage, the inner member and the tripod member can be simplified, and cost reduction for the constant velocity joint can be achieved. Since the graphite steel subjected to austempering treatment transforms into bainite structure, a tough material of high durability can be obtained.
As the graphite steel described above, such a material is used that contains 0.45 to 0.75% of C, 0.4 to 2.0% of Si, 0.2 to 1.0% of Mn, 0.025% or less S, 0.02% or less P, 0.01 to 0.1% of Al, 0.001 to 0.004% of B and 0.002 to 0.008% of N by weight as the basic components, with the rest comprising Fe and inevitable impurities.
Among the elements described above, C is an indispensable element for forming graphite. When the concentration of C is lower than 0.45%, surface hardness achieved by heat treatment becomes too low to obtain a sufficient strength. When the C content is higher than 0.75%, toughness achieved by the heat treatment decreases.
Si is added as a deoxidizing agent and graphitization accelerating agent during the steel making process and, in addition, for the purpose of enhancement of grain boundary. When the concentration of Si is lower than 0.4%, it becomes difficult to graphitize the carbide and the effect of grain boundary enhancement decreases. When the concentration of Si is higher than 2.0%, cold workability (capability to be forged and cut by turning) lowers significantly.
Mn content is required to fix sulphur, included in the steel, in the form of MnS and diffuse it. When the concentration of Mn is lower than 0.2%, hardenability becomes lower (sufficient depth of hardening cannot be obtained). When the concentration of Mn is higher than 1.0%, graphitization is significantly impeded and cold workability deteriorates.
S, existing in the form of MnS inclusion through bonding with Mn, may become the start point of cracking during cold working, and therefore the concentration thereof is kept 0.025% or less. Concentration of P, that precipitates in the grain boundaries of steel and significantly lowers the hot workability, is kept 0.02% or less.
Al, used as a deoxidization agent to remove oxygen included in the steel by being oxidized during the steel making process and reduce the particle size, is contained with a concentration not less than 0.01%. Since a high concentration of oxide lowers the toughness and the oxide may become the start point of crack during cold working, the concentration is kept 0.10% or less.
B and N are added to reduce the graphitizing annealing time through the generation of BN. While addition of B in a concentration of 0.001% or higher is required to achieve sufficient effect of time reduction, the effect of reducing the graphitizing annealing time reaches a plateau at a concentration higher than 0.004%. N is added in a concentration in a range from 0.002% to 0.008% inclusive, in order to turn the B content of 0.001% to 0.004% into BN.
The graphite steel described above includes 0.3 to 1.0 weight percent inclusive of Ni and/or 0.2 weight percent of Mo added thereto. Addition of Ni increases the ductility of ferrite thereby improving the cold workability and strength. Ni content below 0.3% is insufficient for improving the cold workability and strength, while Ni content higher than 1.0% lowers the turning machinability significantly. Addition of Mo improves the toughness, but content thereof higher than 0.2% impedes grahitization.
Graphite steel containing graphite grains of diameters within 15 xcexcm is used. Graphite grains of diameter greater than 15 xcexcm become the start points of cracking and lower the forging performance.
The components of the joint described above are made to have hardness from 50 to 60 HRC inclusive, particularly in the core. When hardness of the core is lower than 50 HRC, sufficient effect of improving the strength cannot be obtained. When the hardness is higher than 60 HRC, toughness decreases. Hardness of the core can be changed by adjusting the austempering temperature and the carbon content in the graphite steel.
When less carbon content is included in the graphite steel, austempering temperature must be lowered while this may cause variations in the surface hardness after heat treatment. In this case, a carburized layer is formed on the surface to increase the carbon content in the graphite steel, before applying austempering treatment.
Surface hardness can be increased to, for example, about 900 Hv and improve the wear resistance, by forming a nitrided layer through diffusion of nitrogen in the surface layer that has been subjected to austempering treatment.
Forming sulfide (such as FeS film) in the surface layer after austempering treatment improves the surface lubrication and the stability of the constant velocity joint operation. The sulfide may be formed either directly on the surface after austempering treatment or after forming the nitrided layer on the surface.
Hardness of austempered surface is generally lower than that of a carburized part. Thus, wear resistance can be improved by using a low friction grease, specifically a grease having a friction coefficient xcexc of 0.07 or lower, for filling the inside of the constant velocity joint. The friction coefficient xcexc can be measured with SAVIN type friction wear tester.
According to the present invention, as described above, since austempering teatment is employed instead of carburizing treatment in the prior art as the heat treatment process for the cage, the inner member and the tripod member, less deformation due to the heat treatment results. This makes it possible to simplify or omit the machining processes such as grinding employed for ensuring the accuracy after heat treatment, while the reject ratio also becoming lower than that of the prior art. Also the double processes of hardening and annealing can be integrated into one process, so that the heat treatment cost is reduced. In order to achieve a hardness of HRC50 or higher while applying austempering treatment to carbon steel, high carbon content is necessary which in turn significantly lowers the workability for forging and machining. However, the use of graphite steel ensures high workability for forging and machining. As a result, manufacturing cost for the constant velocity joint can be cut down through the simplification of the processes.
These and other objects and advantages of the present invention will become clear from the following description with reference to the accompanying drawings.